The present inventor has fostered not only the adoption of TVSS to protect downstream electrical equipment from incoming power line surges--such as from lightning strikes or switching transients--but also the safeguarding of TVSS apparatus from possible catastrophic failure if the current loads become excessive. Applicant's surgeprotective apparatus utilizes varistors or equivalent non-linear resistance means as components in their usual disklike form to shunt surge currents to ground. Prime locations for TVSS apparatus are at the weatherhead, or between the watt-hour meter and its usual socket, or in a wall panel or in packaged adapters downstream of the meter and upstream of electrical equipment to be so protected.
A common feature of packaged forms of TVSS is lack of capacity to carry extreme current densities. Metal oxide varistors can shunt considerable surge currents to ground and thereby protect downstream equipment, but repeated surge conduction increases the likelihood of failure in associated equipment or in TVSS apparatus itself, if cooling time and paths are inadequate. Varistors may get so hot therefrom as to reach a characteristic failure temperature, resulting in loss of physical integrity--perhaps explosively--often with arcing.
A known type of downstream surge-protective device is often packaged in typical polyhedral form. An example of such device is disclosed by Reitz as a "Secondary Arrester" in U.S. Pat. No. 4,439,807. Commercial for many years, such device lacked appropriate fusing.
Rather than to rely upon the installers of prepackaged surge-protective equipment to provide adequate local fusing to protect it from possible overloading and failure, it is preferable to include in a TVSS package effective means to increase its current-carrying capacity, to inhibit its temperature rise, to disconnect it from the power lines if failure becomes imminent, and to extinguish any arc arising from inadequacy of conduction capability, overheating, etc.
Varistors have characteristic failure temperatures at which not only their electrical conduction but also their structural integrity is impaired and at which an electrical arc fire may occur, damaging adjacent equipment nearby and perhaps being more broadly damaging.
The present inventor has pioneered increases in surge capacity and safety of TVSS apparatus by heat-sinking component varistors (in U.S. Pat. No. 4,931,895); by stacking varistor disks in parallel circuit (in U.S. Pat. No. 4,901,187); by using temperature-responsive "thermal" fuses or similar cutoff devices to sense temperature of varistors and to disconnect them from the power lines in the rare but possible event of excessive temperature rise (in U.S. Pat. No. 4,866,560); and by including distributed-resistance fuse links--with or without thermal cutoff means and/or varistor stacking--between power line leads and such varistor (in U.S. Pat. No. 4,907,119).
The present invention relates to extension of such improvements to automatic extinguishment of electrical arcs in surge-protective apparatus, as exemplified in a prepackaged TVSS device suitable for use in a weatherhead, meter adapter, utility panel, downstream plug-in to an electrical outlet, or at other surge-sensitive locations.